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Thermochemical processes I Heat Treatments

Carburizing / Carbonitriding Surface hardening of Steel 2035 2030 2025 2020 2015 2010 2005

High wear resistance

Increased mechanical properties

High stability

Improved fatigue strength

High surface hardness

Improved ductility

Increased impact resistance

High resistance against impacts

mamesta b.v. Spikweien 27 NL • 5943 AC Lomm Tel.: +31 77 308 13 33 [email protected] www.mamesta.nl

heat & surface treatment b.v. Achtseweg Noord 5 Gebouw AL, NL • 5651 GG Eindhoven Tel. +31 40 266 30 00 [email protected] www.h-st.nl

Carburizing / Carbonitriding Surface hardening of Steel What is case-hardening?

interstage cooling or new heating to a new

to partially to case-harden due to appropriate

Case-hardening is one of the thermochemical

material-specific curing temperature. In order

insulation techniques. The repeatability of

processes. During this process the edge

to minimize distortions, it is also possible,

the processes is achieved through continuous

of workpieces is enriched with carbon

instead of directly in cold oil or water, to

monitoring of the process parameters.

(carburizing), or a combination of carbon

quench in oil or in a warm bath.

and nitrogen (carbonitriding) to improve the

The third step is tempering and mainly

Case Hardening Depth

mechanical properties of the edge layer of

serves to diminish the highest stresses in the

The Case Hardening Depth, in short CHD or EHT

the workpiece. Typical case hardening steels

structure and to diminish the grinding crack

(Einsatzhärtetiefe), is the vertical distance of

include a carbon content of 0.10% to 0.25%

sensitivity. If required, a deep cold treatment

surface, where hardness has decreased to a

carbon and are easily machined in soft state.

is possible. In this case, the workpiece is

defined value (core hardness + 50, HV1). The

The alloyed case hardening steels contain

cooled to temperatures far below zero (at least

case hardening depth with its corresponding

chromium, nickel and molybdenum as the

below -80 °C) in order to reduce the residual

border hardness and test load must be agreed

most important elements. These increase

austenite percentage and thus improving the

between the customer and the heat treatment

significantly the hardenability of steel.

dimensional stability after hardening.

company, depending on the material and the

How does case-hardening take place?

Process Variations

standard.

Case hardening typically consists of three steps

In both carburizing and carbonitriding, the

The case hardening depth is measured by

In the first step, workpieces are exposed to a

chemical composition of the workpieces and

means of a destructive research to the product

carbon and/or nitrogen emitting environment.

the structure of their edge layers changed.

or the provided test piece.

This increases the carbon and/or nitrogen

Combination and carbonitriding is performed

percentage at the edge of the workpiece.

in liquid and gas media. Contrary to

The carbon profile and the penetration depth

carburizing, carbonitriding reaches higher

depend on the time at temperature, the

hardness and higher hardness penetration

amount of supplied carbon, the temperature

depths (Case hardening depths) in unalloyed

and the kind of steel used.

steel types. For both variations, different

In the second phase, quenching (hardening)

techniques are available such as in-and-out

takes place. This can be performed directly

furnaces, conveyor-belt furnaces, low pressure

from the carburizing temperature, or after an

installations etc., and further it is also possible

application, in conformity with the applicable

• Higher surface hardness • High toughness • Improved resistance against fraction

Ac3 core Carburizing Ac3 edge

Harden

Temperature

• Higher wear resistance

Temperature

Properties Ac3 core Carburizing Ac3 edge

Harden

• Improved fatigue strength

A

• Partial hardening possible

B

Temper

Temper

Ac3 core Carburizing Harden

Ac3 edge

Time

Temperature

Temperature

Time

Ac3 core Carburizing Ac3 edge

Tough core

Harden

C

D

Temper Time

Hard edge layer Graphical representation of a case-hardened component

Possible temperature-time paths when carburizing

Temper Time

A

Direct hardening

B

Single hardening

C

Hardening after isothermal transformation

D

Dual hardening